The present invention relates generally to drip irrigation tape and its manufacture.
Drip irrigation systems are commonly used in agriculture and horticulture to conserve water and reduce costs. The drip irrigation system comprises lengths of plastic tubing or tape placed near the roots of plants, either above ground or embedded in the earth, the tape having numerous small outlets supplying drops of water continuously to the plants. As well as conserving water, this arrangement provides a more uniform water supply to plants, improving crops and reducing salt accumulation and fertilizer loss in the soil.
Drip irrigation tape is commonly fabricated today from a thin pliable plastic strip folded lengthwise. The two edges are overlapped and joined together to form a flat hollow tape that may be of the order of one inch wide. Under pressure, the tape opens out into a generally cylindrical form to provide the main conduit for irrigation water flowing to crops or other plants under irrigation. The tape also includes a much smaller secondary conduit, usually located along the seam formed by the overlapping side edges of the plastic strip, and connected to the main conduit to form a narrower passageway to reduce the rate at which water is emitted into the soil. The water trickles out of a series of outlets along the secondary conduit. One problem in such tapes is to ensure uniform drip rate along the tape. This problem is particularly acute in cases where extremely long runs of tape are to be used. Currently, the maximum run of tape which can be used from a water supply is around 600 to 700 feet before a relatively large reduction in drip rate will be encountered. This can be a problem in large crop regions to be irrigated.
In my previous U.S. Pat. Nos. 4,722,759 and 4,807,668, a drip irrigation tape is described in which a strip of flexible material is formed with an indented groove extending lengthwise adjacent one side edge of the strip, and the other side edge is folded over lengthwise to overlap the groove and form the main conduit within the interior of the tape. The overlapping side edges are sealed together on opposite sides of the groove to form a seam in which the groove forms a secondary conduit. Spaced inlets are provided between the main conduit and secondary conduit, and spaced outlets lead from the secondary conduit to the exterior of the tape, so that fluid supplied to the main conduit flows into the secondary conduit and leaks slowly out of the outlets into the surrounding soil.
In U.S. Pat. No. 4,722,759 a method of fabricating such tape is described, in which a straight, continuous or segmented groove is formed along the side edge of the strip on a vacuum drum. However, there is some advantage in providing a non-straight flow path along the secondary conduit to create some turbulence in the fluid, tending to shift any debris which might otherwise block the conduit. The tortuous or winding flow path will also reduce the flow rate and allow the dimensions of the secondary conduit to be increased while providing the same drip rate as a corresponding straight conduit, further reduce the risk of debris blocking the channel. Drip irrigation tape with a winding or serpentine secondary conduit is described, for example, in U.S. Pat. No. 4,177,946 of Sahagun-Barragan, U.S. Pat. No. 4,473,191 of Chapin, and in my co-pending application Ser. No. 07/485,778 filed Feb. 22, 1990, abandoned.
There are problems in manufacturing tape with a winding or serpentine secondary conduit. In Sahagun-Barragan, the tape is formed from two sheets, one of which is embossed to form a winding channel. In Chapin, the winding channels are formed by a pair of paired ribbons of adhesive or hot melt polymer between two flat sheets of plastic film, causing problems in maintaining uniformity of the channel cross section along its length. In my co-pending application Ser. No. 07/485,778, now abandoned, referred to above, a method and apparatus for manufacturing irrigation tape with a winding secondary conduit is described, in which opposing drums have mating indented groove and projecting mandrel formations for forming the channel, the shape of the mating groove and mandrel corresponding to the desired conduit shape. One problem with this is that the two drums must be synchronized for proper operation.
Other problems inherent in existing drip irrigation tapes are that the drip outlets often become blocked with plant roots growing over or into them, for example, and that although winding secondary conduits have advantages over straight conduits, they are very difficult to make.